1. Technical Field
The present invention relates generally to the testing of integrated circuit chips and, more particularly, to an optical triggering system and method for synchronizing the optical test of an integrated circuit chip with its operation.
2. Description of Related Art
The Picosecond Imaging Circuit Analysis (PICA) technique is widely used to test modem integrated circuit chips due to its low invasiveness and high time resolution. The PICA testing technique is based on the collection of near infrared (NIR) light emitted by hot-carriers in the transistor channel of the chip, and has been demonstrated to be an invaluable method for detecting timing-related faults in backside-packaged Ultra Large Scale Integration (ULSI) circuits. Many innovative applications of the PICA technique have been developed in past years based on the detection of light emission due to off-state leakage current, oxide tunneling current and carrier recombination, thus leading to new application areas and more contributions to quick turn around times for circuit debugging and defect isolation.
Because of the important advantages offered by the PICA technique over other testing methods, which are often invasive and may lead to damage of the chip under test (CUT), a significant amount of effort has been devoted in recent years to the development of innovative detectors to compensate for the challenges related to the continuous reduction in voltage and size of modem ULSI circuits. Two detectors that have demonstrated significantly improved quantum efficiency (QE) in the NIR region of the spectrum are the Superconducting Single-Photon Detector (SSPD) and the InGaAs Avalanche Photo-Diode (APD) which offer a single-point detection capability. A third detector, the MEPSICRON Photo-Multiplier Tube marketed by Quantar Technology Incorporated of Santa Cruz, Calif., provides an imaging capability; and, in fact, permits the parallel acquisition of waveforms from hundreds of transistors in the same field of view.
Besides reduced emission intensity, modem ULSI circuits also provide a challenge related to the timing performance required to construct the optical signals emitted by the circuit during normal operation. A measure of such a capability is the Full-Width at Half Maximum (FWHM) of the detected optical signal when an ideal light impulse is emitted by the circuit (the larger the FWHM, the worse the timing performance). The FWHM of certain measurements is given by the quadratic sum of many different components ranging from detector resolution, front-end jitter, circuit jitter, optical signal dispersion, etc.
Fundamental for the PICA testing technique is the extraction of a trigger signal, synchronous with the operation of the integrated circuit that is used to reconstruct the histogram of the photon arrival time. Any jitter in the trigger signal, compared to the integrated circuit, limits the timing performances of the measurements. In particular, in the PICA testing system, there is an electrical connection between the PICA system and the CUT along the trigger path. When a high speed electrical trigger signal is extracted from the chip, it may be affected by jitter comparable with that of the detector (˜50-ps FWHM) used in the PICA system; thus limiting final resolution. This limitation is due mainly to the resistive/capacitive load and quality of the electrical connection between the chip and the PICA system along the trigger path. Also, for some PICA applications, an electrical trigger signal cannot always be obtained from the CUT.
There is, accordingly, a need for a triggering mechanism for synchronizing the test of an integrated circuit chip with its operation that has reduced jitter and a higher frequency rate resulting in a more accurate test of the integrated circuit chip.